1. Field of the Disclosure
The present disclosure relates to an amplifier circuit that inputs thereto, in a high impedance, a weak signal from a sensor or the like and amplifies the weak signal.
2. Description of the Related Art
Generally an instrumentation amplifier is known that is used as a circuit that inputs thereto, in a high impedance, a weak signal generated by a sensor or the like and amplifies the weak signal. In Japanese Unexamined Patent Application Publication No. 7-46046, describes a technology for amplifying an output signal from a magnetic head by using an instrumentation amplifier.
FIGS. 4A and 4B are diagrams each illustrating a basic configuration of an instrumentation amplifier. As illustrated in FIG. 4A, the instrumentation amplifier is configured by using two operational amplifiers U1 and U2. In the operational amplifiers U1 and U2, feedback resistors R11 and R12 are connected between respective output terminals thereof and respective inverting input terminals thereof, and the respective inverting input terminals of the operational amplifiers U1 and U2 are connected to each other via a resistor R13. If a signal is input to non-inverting input terminals of the respective operational amplifiers U1 and U2, an amplified signal is output, as a differential signal, from outputs of the respective operational amplifiers U1 and U2.
Since the non-inverting input terminals of the respective operational amplifiers U1 and U2 function as signal input terminals, the instrumentation amplifier has a very high input impedance. By matching resistance values of the feedback resistors R11 and R12 with each other, a high common-mode signal rejection ratio (CMRR) is obtained.
In addition, a gain of the instrumentation amplifier is adjustable by a resistance value of the resistor R13, and the high input impedance or the common-mode signal rejection ratio is not influenced by the resistance value of the resistor R13. In a case of a general differential amplifier circuit in which an operational amplifier is subjected to negative feedback by using a resistance element, both a gain and an input impedance change in accordance with a resistance value of the resistance element. However, in a case of the instrumentation amplifier, a high input impedance is maintained independently of a setting of a gain based on negative feedback.
FIG. 4B is a diagram illustrating a configuration of a general operational amplifier used for the instrumentation amplifier. The operational amplifier illustrated in FIG. 4B includes an input amplifier stage (Q21 to Q24 and 101) and an output amplifier stage (Q25, R14, C11, and 102). The input amplifier stage includes a differential pair (Q21 and Q22) configured by two transistors, a current mirror circuit (Q23 and Q24) connected, as a load, to a drain side of the differential pair, and a constant current circuit 101 that applies a constant current to a source side of the differential pair. The output amplifier stage includes a transistor Q25 configuring a source-grounded amplifier circuit, a constant current circuit 102 connected, as a load, to a drain of the transistor Q25, and A phase compensation circuit (R14 and C11) that limits a band of the output amplifier stage for the stability of a feedback system.
In a case where a direct-current or low-frequency minute signal is amplified with a high gain, there is a problem that a flicker noise (a 1/f noise) that increases with a decrease in frequency is required to be reduced. The flicker noise is mainly generated by a transistor. Therefore, if many transistors are used for an input stage, the influence of the flicker noise grows. In a case of the operational amplifier illustrated in FIG. 4B, two transistors are used, as a differential pair, for the input stage. Therefore, if this is used for the instrumentation amplifier illustrated in FIG. 4A, four transistors turn out to be used for an input stage as a whole. Accordingly, compared with a case where a single operational amplifier is used, the number of transistors in the input stage increases in the instrumentation amplifier, and there is a problem that the flicker noise is considerable.
The present invention is made in view of such a situation and provides an amplifier circuit able to maintain a high input impedance while not being influenced by a setting of a gain based on negative feedback and able to reduce a noise caused by transistors in an input stage.